Since their discovery in 1994, a strong observational effort was done to find FGs. Despite their supposed frequency, more than 10 years were needed before producing reasonably large catalogues of some tens of candidates. In this sense, one of the main obstacles for the community was to agree on a practical definition. In fact, at the beginning the search was limited to isolated galaxies surrounded by an X-ray halo, but this definition was too loose [1]}. More rigorous definitions, involving photometry, spectrocopy and X-ray data, were proposed and are actually used. We will discuss them in Sect. REF . However, these rigorous definitions had important observational limitations. The result of these constraints was that fossil samples were not selected in a homogeneous way and, in Sect. REF , we will discuss the strength and weaknesses of the different approaches adopted in the literature.
